The present invention relates to numerical data transmission systems and more particularly coherent demodulation binary-signal modulators-demodulators for transmission on millimeter waves.
The use of coherent demodulation modulators-demodulators enables the need for frequency stability to be overcome, which is difficult to obtain in millimeter waves, which is required of differential-phase demodulation devices so as to keep, during link-up, an optimum signal-to-noise ratio. But coherent-phase demodulation requires restitution of the carrier at reception.
So as to avoid the cost of generating and transmitting an appropriate signal for this restitution, it is known, at transmission, to frequency modulate an oscillator from data to be transmitted transformed after coding.
A first type of coding uses Walsh I type signal coding (this denomination corresponding to the fact that these signals are identical to the first basic vector of break-down into Walsh functions).
Thus, from two quadrature components of the modulated signal, it is possible to readily demodulate one of the components carrying the information properly speaking in the form of a phase-jump modulation, the other component carrying the carrier and timing information, which allows after recovery of the carrier coherent demodulation to be achieved.
The disadvantage of this first type of coding associated with frequency modulation comes from the fact that a modulation density appears in the spectrum about the central frequency, which results in the creation of phase variations on the carrier recovered in the receiver by filtering or phase-dependent loop.
A second type of coding uses a Walsh II type signal coding (second basic vector of the break-down into Walsh functions).
This type of coding has the advantage of leading to a frequency modulated signal which comprises a central line free from modulation, but requires a larger frequency band, which is an impediment especially when high flows of information are to be transmitted.
The present invention aims at substantially reducing the disadvantages of these two solutions while keeping their respective advantages to provide coherent modulation of frequency modulated signals, the spectrum of these signals not presenting any modulation density about the carrier frequency, which facilitates the recovery of the carrier in this spectrum and, at the same time, the width of the effective band of the spectrum remaining fairly wide, which is important especially for high information flows.